Thermal expansion behavior of nylon 6 nanocomposites

The linear thermal expansion behavior of nanocomposites was measured and compared with the theory by Chow. Nanocomposites were prepared by melt mixing organically modified montmorillonite with high and low molecular weight (LMW) grades of nylon 6 using a twin screw extruder. Thermal expansion measurements were made on samples taken from injection-molded Izod bars in the flow direction (FD), transverse direction (TD), and normal direction (ND). Addition of clay reduces the thermal expansion coefficient in both FD and TD while an increase is seen in ND; FD has a lower thermal expansion coefficient than TD. The latter suggests non-uniform orientation of exfoliated platelets about FD, since perfect alignment of disk-like platelets in an isotropic matrix must yield identical expansion coefficients for both FD and TD. High molecular weight (HMW) nylon 6 nanocomposites resulted in lower thermal expansion than LMW nylon 6 nanocomposites at the same organoclay content. This difference was attributed to the higher aspect ratio of particles (better exfoliation) in HMW nylon 6 nanocomposites. Thermal expansion coefficients predicted from a theoretical composite model were compared to the experimental data. Platelet aspect ratios deduced in this way were found to be dependent on the specimen direction, which further suggests imperfect orientation of particles about FD. Morphological analysis by transmission electron microscopy support the conclusions drawn above.